Traveling-wave tube



July 2, 1957 s, sENslFER 2,798,183

TRVELING-'WAVE TUBE 2 Sheefs-Sheet J Filed Nov. 29.1954

. July 2, 1957 s. sENslPER.

TRAVELING-WAVE TUBE 2 Sheets-Sheet 2 Filed Nov. 29. 1954 United States Patent O i 2,798,183 TRAVELING-wAva Tuna Samuel Sensiper, Los Angeles, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Application November 29, 1954, Serial No. 471,660

6 Claims. (Cl. S15-3.5)

The present invention relates to traveling-wave tubes having improved unidirectional attenuationV characteristics, and more particularly to a traveling-'wave "tube including apparatus utilizing the characteristics opfrferrite materials capable'of attenuating the energy of'a wave propagated in one direction bythe wave propagating structure without substantially affecting the energy of waves propagated in the opposite'direction.

The traveling-wave tube of the present invention incorporates a slow-wave propagating structure of thetype disclosed in a copending application for patent Serial No. 450,987 entitled, High Power Microwave Tube,led on August 19, 1954, by Charles K. Birdsall. This slowwave structure comprises a series of periodically spaced conductive rings of uniform diameter alternately con-V nected together with longitudinal conductors at 'diametrically opposite points on the rings. An analysis of the wave propagating characteristics of this structure shows that the portions of the wave contiguousA to the structure and in quadrature with the longitudinal conductors are substantially circularly polarized whereas the portions of the wave along the longitudinal conductors are linearly polarized.

In accordance with the present invention, ferrite members are disposed contiguously along the above wave propagating structure only in the region where the propagated wave is substantially circularly polarized. A magnetic field is produced through each ferrite member in a direction to effect unidirectional attenuation of .an electromagnetic wave propagated by the structure. That is, the ferrite material has little or no effect on a-wave propagated in a forward direction but attenuates a wave propagated in a backward direction. The absence of ferrite material along the longitudinal conductors minimizes attenuation of the forward wave thereby greatly enhancing the utility of the device. In accordance with an alternate embodiment of the invention a plurality of air gaps is interposed longitudinally along the ferrite members to increase their reluctance to the magnetic field generally employed to focus or constrain the electron stream. Also, in another embodiment of the invention, the ferrite members are divided into predetermined lengths along the longitudinal axis of the propagating structure and different magnetomotive forces are developed across the different lengths of ferrite so as to attenuate a `wave propagated in a backward direction over a broad range of frequencies. i a f At the present time it has been proposed to provide a unidirectional slow-Wave propagating structure by disposing a ferrite helix contiguously about a conventional conductive helix. The higher relativeapermeability of the ferrite material causes a portion of-the longitudinal magnetic field normally employed to focus the electron stream to be shunted through the ferrite helix. A por-` tion of the magnetic iield is thus directed circumferen tially-around the conductive helix thereby eiectuating unidirectional attenuating characteristics lin the ferrite materiel. 1 It has been, found that a device of this. type has `atented July 2, 1957 limitations in that the complex configuration of the cef ice ramic ferrite material makes fabrication difficult and, in

addition, the magnetomotive force necessary to operate the ferrite at gyroresonance for the higher microwave fre- VAquencies', i. e. the degree of saturation necessary to produceunidirectional attenuation, is normally substantially more than that necessary for focusing the electron stream. lt is therefore an object of the present invention to provide a traveling-wave tube incorporating an improved slow-wave propagating structure having unidirectional attenuating characteristics.

, Another object of the present invention is to-provide a traveling-wave tube incorporating apparatus including ferrite members of simple configuration capable of attenuatingan electromagnetic wave propagated by a conl trawound helix in a backward direction without vsubstantially affecting a wave propagated in the forward direction.

Still another object of the present invention is to provide a traveling-wave tube incorporating a contrawound helix together with apparatus capable of dissipating energy propagated by the helix in one direction over a wide range Yof frequencies without substantially affecting wave energy propagated in an opposite direction by'the helix.

The novel features which are believed to be character-y istic of the invention, both as to its organization and method of operation, togetherwith further objects and advantages thereof, will bevbetter understood from the following description considered in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of example. ,It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

l the contrawound helix included in 4the i Fig. 1 is a diagrammatic sectional View of the tube of the invention together with associated circuitry; i

Fig. 2 is a cross sectional view taken on line 2 2 of Fig. l; r

. Fig. 3 is a schematic diagram of the current flow along tube shown in Fig. 1; l

, Fig. 4 is a cross sectional view taken on line 4-4 of Fig. 1 of the contrawound helix shown in Fig. 1 showing the magnetic eld configuration for the current flow of Fig. 3;

Figs. 5 and 6 show vectors illustrating the circular polarization characteristics of the wave propagated by the contrawound helix shown in Fig. 4;

Fig. 7 is a graph showing a typical permeability characteristic for a ferrite material in the gyroresonance region;

Fig.. 8 is a graph illustrating the relative unidirectional attenuating characteristics of the device of Fig. l;

Figs. 9 and l0 show modifications of the unidirectional attenuating apparatus of the device of Fig. 1; and

larged portion 12 there is disposed an electron gun 14 capable of producing a hollow cylindrical electron stream. Electron gun 14 includes an annular cathode 16 with a heater 18, a focusing electrode 20, and an accelerating anode 22, the electrodes 20 and 22 being provided with I apertures in register with the electron emitting annular surface of cathode 16 to allow passage therethrough of Y the electron stream.

The electron emitting surface of cathode 16 is disposed concentricallyabout and in a plane normal to Ithe longitudinal axis of envelope 10. Its heater 18 is energized by a connection across a source of potential, such as a battery 24. The focusing electrode 20 provides an inner and an outer surface of revolution disposed adjacent to and at an angle of approximately 67.5 with the-electron emitting surface of cathode 16, as shown inthe figure, to provide for focusing the electron stream. Cathode 16 and focusing electrode 20 are connected together andare, in turn, connected to the negativeterniinal of aVV battery 26, the positive terminal of which is connectedvto ground. The magnitude of the potential provided by battery 26 maybe of the order of 31000 volts. Accelerating anode 22 is disposed in a plane normal tol the vlongitudinal axis of envelope ltl'and to the 'right of 'focusing electrode -20 as viewed inthe drawing. Anode 22 smaintained at a` substantially fixed potential that is of the order :of 300 volts positive with respect Yto Aground by means of a bonnection to the positive terminal of a battery 2-8, the negative'terminal of which is connected tofground. l i e i A solenoid 30 is disposed concentrieally about.v the ci'nplete length of the elongated portion 11 of envelope 1:0. An apprpriate direct current is made to `tlow through "the slenoid y30 by means of a connection across abattery 32 so as to produce a magnetic eld ofthe order of 600 gauss extending longitudinally along the tube. Thepu'rpose of this magnetic field is to yfocus or constrain theelec'tr'on stream along a predetermined path Y`that is c'oexten'sive withtlie ,active length of thev tube.'

A collector electrode 34 is disposed at the extremity Y fthis'path farthest from the electron gun 14 to intercept and collect the electron stream. Collector electrode 34 is maintained at a potential of the order of 200 volts positive 'with respect to ground by means of a connection therefroin`to the positive terminal of a battery I36,'the negative terminal of Vwhich is connected to ground.

Disposed concentrically around and contiguously along the Vpath'of the electron stream and extending from the electron gun 14 to 4the collector electrode 34 is a contrawoun'd helix 40 comprising ferrules 41, 42 at its extremities and a plurality of annular rings 44 of equal diameter spaced periodically therebetween and connected together at `Adiamet'r'ically opposite points by longitudinal conductive members 46. VHelix -40 is preferably fabricated from a single nonrnagnetic tubular piece of metal such as, for example, tungsten or molybdenum.

A'n 'equipotential region between the electron gun 14 and the helix V40 is .provided by a conductive coating 48 disposed therebetween on the inner surface of envelope 10. Conductive coating 48 may be provided, for example, by a colloidal suspension 'of graphite in water known commercially as aquadag Both helix 40 and "conductive 'coating48 are maintained' at ground potential by suitable connections to ground.

Contrawound helix 40 is energized with an electromagnetic signal to be amplified by means of an input wave guide section S0 which symmetrically encloses the elongated portion '11 of envelope 10 coextensive with the longitudinal conductive member 46 of helix 40 nearest the electron gun 1`4. Input waveguide section 50 is terminated so as to produce a voltage Vmaximum at 'a point coinciding with the longitudinal conductive member 46' and, in addition, has 4a sleeve 51 extending contiguously 'along the envelope 10 for a distance necessary toV produce a virtual shorting plane -at the 'inner surface of waveguide section 5'0 nearest the electron gun 14. Similarly, an output from the tube is provided by-anoutput wave guide section 52 whichsymmetrically encloses elongated portion 11 of envelope 10 coextensive with the longitudinal conductive member 46`farthest along the path offthe electron stream. Waveguide section V52 is terminated Aso as to optimize the energy transfer thereto from the contrawound helix 40. Also, a sleeve 54 extends contiguously along envelope 10 along ferrule 40 for a distance necessary to produce a virtual shorting plane on the inner surface of the waveguide farthest from electron gun 14 in the same manner as on waveguide section 50.

Disposed Valong the outer circumference of contra- Wound helix 40 in quadrature with the longitudinal conductive members 46, 46', 46" are ferrite slabs 60 and 62 composed of, for example,ferrite material known commercially as Ferramic A, Ferramic G, Ferramic R-1, Ferroxcube 104 or Ferroxcube 106. These slabs are shown in partial section in Fig. 1 and in cross .section in Fig. 2. Referring to Fig. 2, the Vheight'of the ferrite slabs 60, 621is ofthe same order of magnitude as the contrawoundhelix 40. Ferrite slabs 60 and 62 are of the order of 0.014 inch thick and spaced 0.020 inch from the outer periphery of the helix 40. As shown inthe present embodiment, the ferrite slabs 60, 62 arc disposed inside the evacuated envelope 10. In this event, it may be necessary to seal the exterior of the ferrite members with a thin layer 4of material such as, for example,'gl a'ss in order to maintain a suitable vacuum within the envelope 10. A method and apparatus for accomplishin'g this is disclosed in a copending application for patent Serial No. 430,841 entitled, VitreousCoated Magnetic Material vtiled on July 19, 1954, by Arthur H. Iverse'n. Alternatively, it is apparent that the ferrite slabs 60, 62 may be located outside of the evacuated chamber 'provided by envelope 10.

' In accordance with the present invention, transverse magnetic fields of Vopposite directions are produced through slabs 60 and 62. To accomplish this 'an iron 'ore 64 having a length that is coextensive with that of ferrite-slabs 60, 62 -is provided. This core is disposed on top-of the elongated portion 11 and has pole pieces positioned in register with the edges of the ferrite slabs l60, 62V. f Iron core 64 is laminated in a direction transverse tothe longitudinal axis of envelope 10 so las to present ahigh reluctance path to the magnetic field used for focusing the electron stream. In order to complete the magnetic path through ferrite slabs 60, 62, an iron core 66, similar to the iron core,64, is disposed across the opposite edges thereof. A solenoid 68 is disposed about the core 64 and connected across a variable voltage s'ou'rce v70. Source -70 is adjusted so that there are sufiicient ampere-turns about the core 64 to develop a magnetomot'ive across each of the ferrite slabs 60 Vand 62 of the 4order of 1500 oersteds, depending on the frequency and type of ferrite used.

During theoperation of the device of the present invention,a'n electromagnetic wave is propagated by the contrawound helix 40. vAs in l,any propagating structure, in

rdrto-causeurrents to fi'ow it lis necessary to provide aboundar'y for an electrodynamic field. Referring now to '-Fig. `3 there is shown a ysectional schematic diagram of 'contrawound helix 40 with arrow indicating the insta'ntaneous current tlow thereon for -an electromagnetic wave `propagated in the forward direction along the path of Vthe electron stream. As-shown in this diagram, thelcurrent ilow divides and flows -in parallel about each half of the annular 'rings 44 -and combines when flowing along "the longitudinal conductive members 46. This current flow generates the magnetic/field portion of the electromagnetic wave propagated by theV contrawound helix 40.

. t'I`o"illustrate 'more clearly the magnetic-'field portion of the waves vnearest the ferritesla'bs'60, 62"refe`re`nce is inade to'Fg. '4,"which shows section 4 4 'of Fig. 1. This is sa cross section view of thefer'ri'teslabs i60, `62-and contrawolind-helix 40 taken 'inaf'horizontal plane," a's viewed inthe drawing, that 'passes through-the points of -helix 40 nearest thelferriteslabs. :Inthis gure,"the=instantane ous magnetic iieldfor a `current ow v'indicated in Fig. 3 is yshown by dash Ilines v70 furthe-'side nearest yferrite -slab I15.0.' arid dash-lines v7.2 Aforlthe-sidenearest-ferrite slab 62. In the operation f the tube of the present invention, magnetic elds of this type are propagated along the path of the electron stream by the contrawound helix 40 at a velocity designated as the phase velocity of the wave.

In order to consider the characteristics of the magnetic field represented by dash lines 70, the field will be viewed from a single point within the ferrite slab 60 such as, for example, at point A. It is seen that the magnetic field for the Vparticular instant shown may be represented by vector E as illustrated in Fig. 5. As the wave progresses along the helix 40, point -A will be successively cut by other points of the magnetic field where the magnetic field may be represented, respectively, by

B within slab 62 sees successive magnetic vectors e, f,

E and illustrated in Fig. 6. As before, these vectors may be represented as a single vector rotating in the counter-clockwise direction at an angular velocity wz.

In accordance with the present invention, the magnetic field through ferrite slabs 60, 62 is directed in a manner such that the magnetic field components of the wave being propagated in the forward direction will be negative circularly polarized. In that the magnetic field intercepting both ferrite slabs 60, 62 rotates in opposite directions, it is apparent that in order to have components of the propagated Wave on both sides of helix 40 polarized in the same .nrection, it is necessary that the direct current magnetic field through ferrite slab 62 be in a direction opposite to that through slab 60.

As is generally known, the permeability characteristics for positive and negative circularly polarized waves of ferrite material are different. More particularly, the permeability for the negative circularly polarized waveis substantially constant, Whereas the permeability for the positive circularly polarized wave goes through resonance for changes in frequency or magnetic field. During resonance the power dissipated in the material increases substantially, thereby making the permeability for a positive circularly polarized Wave a complex quantity. A typical permeability characteristic for a ferrite material is shown in Fig. 7 wherein ,nrepresents the permeability for the negative circularly polarized wave and p.+=,u'-]y. represents the permeability for a positive circularly polarized Wave. As illustrated in this figure, lines 80, 82 and 84 represent the variation in M y.' and n", respectively, versus frequency f-or circularly polarized waves. The magnitude of n", i. e. the imaginary part of ,ti-t, determines the extent to which the positive circularly polarized wave will be attenuated. As shown in the figure, maximum attenuation of the positive circularly polarized wave occurs at the resonant frequency. The point at which this resonance occurs is both a function of the frequency of the circularly polarized Wave as seen by the ferrite material and the strength of the direct current magnetic field across the ferrite.

Thus, in the operation of the -device -of the -present invention, the current through solenoid 68 which determines the direct current magnetic field is adjusted by means of variable potential source 70 to produce a magnetic field within the ferrite material corresponding to resonance at the operating'frequency.

In operation yof the device of the present invention, `the function of the ferrite slabs 60, 62 is to attenuate electromagnetic wave energy being propagated by the contrawound helix 40 in the direction from the collector 34 to electron gun 14, i..e.in the backward direction, Without appreciably -attenuating waves being propagated in the forward direction. The relative attenuation characteristics for a forward and a backward wave for the device of the present invention lare shown in Fig. 8 as lines 90 and 92, respectively. The attenuation of a Ibackward wave in decibels is approximately ten times that of a forward Wave. On the other hand, if similarly magnetized ferrite slabs were to be disposed contiguously along the longitudinal members 46 of contrawound helix 40, the relative attenuation in decibels of a forward wave would be approximately one-half that of a backward wave. In this case the relative attenuation of a forward wave is represented by dashed lines 94 in Fig. 8. Thus-it is apparent that if the ferrite slabs were to be disposed along the longitudinal members 46 of helix 40, the utility of the device of the present invention would be destroyed.

As shown in Fig. 9, an alternative embodiment of the present invention employs a plurality of ferrite slabs 96 in lieu of either one or both of the ferrite slabs 60, 62 shown in Fig. l. The ferrite slabs 96 are `disposed at uniformly spaced intervals, as shown in Fig. 9, and are separated by either air gaps or other low dielectric-constant nonmagnetic material so as to offer a high reluctance path to the longitudinal magnetic eld produced by a solenoid 30 to focus the electron stream. The higher reluctance path presented to the longitudinal magnetic Ifield =by ferrite slabs 96 minimizes distortion of the transverse magnetic field therethrough produced by solenoid 68 and thus improves the unidirectional attenuation characteristic of the tube.

In another embodiment of the present invention, attenuation of the backward wave over an extended range of frequencies is achieved. As previously mentioned, attenuation of the positive circularly polarized components occurs at a resonant frequency. The exact frequency at which this resonance occurs for a given ferrite material is determined by the direct current magnetic field through the ferrite. The attenuation at'each resonant frequency, however, is not necessarily the same. Accordingly, referring to Fig. l0 ferrite slabs 98 are employed in lieu of either one or both of the slabs 60, 62 in the device of Fig. l. Ferrite slabs 98 comprise, for example, sections 100, 101 and 102. In accordance with the invention, different magnetomotive forces H1, H2 and Ha are developed, for example, across the sections 100, 101 and 102,'respectively, so that maximum attenuation for the sections occur at frequencies f1, f2 and f, as shown in Fig. l0. These magnetic fields may be produced, for example, by means of solenoids 103, 104 and 105, respectively. The length of each of the sections 100, 101 and 102 is adjusted so that the attenuation at each of the above frequencies represented by lines 106, 107 and 108, respectively, is the same. Further, the magnetomotive forces H1, H2 and H3 are chosen to cause the resonant frequencies f1, f2 and f, to occur at appropriate intervals so that the composite attenuation represented by dashed line 109 of the sections 100, 101 and 102 is substantially constant within the frequency range from f, to 72,. It is apparent that this also may be accomplished by using different ferrites with substantially equal magnetomotive forces. Under normal circumstances, it is desirable to have the frequency range f1 to f3 coextensive with the gain characteristic of the tube so as to prevent self-oscillations at frequencies where the tube still provides substantial gain but has a poor impedance match to its output circuit.

What is claimed is:

1. A traveling-wave amplifier tube comprising means for producing an electron stream; a contrawound helix disposed concentrically about and contiguously along a predetermined path for propagating an electromagnetic signal Wave therealong in a forward direction, said contrawound helix including a plurality of equal-diameter conductive annular rings disposed periodically along said rpath, vfand a longitudinal vconductive 'memberldisposedl between teach adjacent Vpair 'of 4rin'gs,1said longitudinal conductive `members interconnecting alternate pairs tof vcorresponding diametrically'opposite points on the ycircumference of each-of said rings; means for directing said electron stream :along ,said .predetermined path in 'said Yforward direction, thereby #tofarnplify said ielectromagnetic signal' Wave; 'output means coupled itoisaid helix, whereby a portion ofthe :energy oftisaidsamplifed electromagnetic :signal 'wave-is reflected and .propagated lback along s'atid'ipredetermined path-in abackward direction; a member composed of ferrite material'having a surface disposed leontiguously along said contrawound helix lin quadrature withthe longitudinal A"conductive members thereof-pand lmeans for producing amagnet-ic field? tra-nsverseit'o the longitudinal axis-of -saidlhelix through 4said member composed offerrite'material, -said magnetic `field having ain i intensity to produce resonance in the permeability cliaracteris'tic-ofsaidferrite 'material `at the frequencyjof =said vv-ave, l'th'e'reby dissip'ati'ng the portion of the energy of `said*electromagnetic signal wave `being propagated along saidpredetermined Lpath in said lback Ward directionwithout substantially affecting Vthe magnitudeo'f said wave being propagated `along said ypath in said-forward direction.

' 2. :Atraveling-wave-amplifier tube-comprisingmeans for 'producing an electron stream; a-contrawoundhelix disposed -concentrically a-boutand conti'guously a-longa predetermined path for rpropagating electromagnetic waves ltherealong ina Vforward direction, said contrawound .helix including a plurality ofequal-'diameter conductive annular rings disposed periodically along said path, and a longitudinal conductive member disposed between each adjacent pair of rings, said longitudinal conductive-members interconnecting alternate pairs of corresponding diametrically opposite points on the circumference of veach of said rings; means Yfor directing said electron stream along said'predetermined path in said forward direction, thereby to transfer energy from said electron stream to said electromagnetic waves; output means'coupled to said helix, whereby a portion of the energy of said electromagnetic waves is reflected and propagated back along said predetermined path in a backward direction; a segment composed of ferrite material having a surface disposed adjacent said contrawound helix in quadrature with the longitudinal conductive members thereof; and means for producing a magnetic field transverse to the longitudinal axis of said helix-through said .segment composed of ferrite material, said magnetic field having an'intensity to cause said ferrite material -to dissipate the'energy of said electromagnetic waves propagated along said predetermined path in a backward direction without substantially affecting the magnitude -of said electromagnetic 'waves being propagated along said path in a forward direction.

3. The traveling-wave amplifier tube as defined in claim 2 including a plurality of said segments composed of ferritematerial, said segments being 'disposed at lintervals along said contrawound helix to increase the reluctance presented to a magnetic field along the longitudinal axis of said'contrawound helix. n l

4. A traveling-wave amplifier tube 'comprising means for producing an :electron strearm'a contrawound helix disposed concentrically about and contiguously along a predetermined "path Yfor propagating electromagnetic Waves therealong in a forward direction, said contrawound helix including a plurality of equalfdiameter conductive annular rings disposed periodically along said path, fand a longitudinal conductive 4member disposed between Veach adjacent pairo'f rings, said longitudinal conductive members 'interconnecting alternate pairs .'of corresponding diametrically opposite'jpoints on 'the ,ciroumference ofweach.y oftsaidy rings; means for xiirectingsaid electron-stream along :said predetermined .pathin said forward fdirectionthereby to transfer 'energy from "said electron 'stream to said electromagnetic waves; l'output means'cou'pled to tsaid'helix, Awhereby azportion of the energy'of said`elec'tromagnetic waves is reflecteed'and propagatedfback along said?. predetermined path in a backward direction; a plurality of segments composed of ferrite material having surfacesy disposed `contiguous along said contrawound helixlin quadrature with the longitudinal conductive `members thereof; and means'for producing magnetic fields transverse to the longitudinal iaxis of said helix and 'of 'different predetermined intensites through each lof`said"seg'r'nents therebyto dissipate nergy'f said electromagnetic 'waves lpropagated Yalong 'saidpre'determined path -in abackward `direction Aover abroadvrange of frequencies without substantially afect'ing'the' magnitude of said 'electromagnetic 'waves being Ipropagated along said path in a forward direction.

5. The travelingwave amplifier tubeas defined in claim 4`whereinthe2s'ur'faces of sai'dplurality'of segments composed offerritefmaterial disposed contiguously along said contrawound Ihelixfare each ofv'predetermined length to produce Gunifor-'m ydissipation of energy throughoutlsaid broadt range-of frequencies,

6;-Atraveling-'wave amplifier Itube comprising means for :producingian electron stream; a contrawound helix disposedconeentrically about and-contiguously along a predetermined path for 'propagating an electromagnetic signal waveftlerea'long in a forward direction, said contra- Woundfhelix including Ya plurality of equal-diameter conductive @annular vrings disposed periodically along said 1`iath,gandt la"lon'gitudinal conductive member disposed `be tweeneac'h'adjacent pair l'of ringsfsaid longitudinal conductivem'enfibers'interconnecting alternate pairs of corresponding `Vdiametr-ically lopposite lpoints lon -t-he circumference of each of said rings; means for directing said electron 'stream along lsaid predetermined path in 'said forwardfdirection, thereby to amplifyV saidelectromagnetic signal wave; output means-coupled'l to said helix, whereby a portionof theenergy of said 'amplified electromagnetic signal #wave fis reflected and propagated Vback along said predetermined path'in a'backward direction; first and second members composed of ferrite material, said first and second membershaving surfacesdisposed contiguouslyy along-fbothsides of'the'contrawound helix in quadrature with the 'longitudinal conductive-members thereof; and means V-for producing magnetic fields in opposite directions throughsaid iir-stand second members and transverse to the longitudinal ofsaid helix, said magnetic fields havingan intensity to produce-resonance-in the permeability characteristic'ofsaid ferrite material at'the frequency of said wave, thereby dissipating the portion of the Venergy of said Vlelectromagnetic signal wave being propagated along 'said lpredetermined'pathl in said backward direction withoutlsubstantially affecting'the magnitude of 4said wave being propagatedjalorrg said Vpath in saidforward direction.

Pages 1077-1080, Electricalv Engineering for December $1949.

Pages 'Z2-'27, ,'Bell 'Sys., Tech., Journal for January Y1952. Pages S16-817, Journal of Applied Physics for Junev 1953. 

